1. Field of the Invention
This invention relates generally to the immunotoxin gelonin and, more specifically, to the molecular biology of gelonin, including a process of a synthetic gene for gelonin.
2. Description of the Related Art
One current interest in cytotoxic substances involves their potential use to specifically target tumor cells. The plant toxin gelonin has received such consideration. Gelonin is a glycoprotein (M.W. approximately 29-30,000 Kd) purified from the seeds of Gelonium multiforum. Gelonin belongs to a class of potent ribosomal-inactivating plant toxins. Other members of this class of ribosomal-inactivating plant toxins are the chains of abrin, ricin and modeccin. Gelonin, like abrin and ricin, inhibits protein synthesis by damaging the 60S sub-unit of mammalian ribosoms. Although the A chain of ricin (RTA) has been popular for use in immunotoxins, gelonin appears to be more stable to chemical and physical treatment than RTA (Barbibieri et al., Cancer Surv. 1:489-520 (1982)). Furthermore, gelonin itself does not bind to cells and therefore is non-toxic (except in high concentrations) and is safe to manipulate in the laboratory. The inactivation of ribosomes is irreversible, does not appear to involve co-factors and occurs with an efficiency which suggests that gelonin acts enzymatically.
Gelonin and ricin are among the most active toxins which inhibit protein synthesis on a protein weight basis. Gelonin is 10 to 1000 times more active in inhibiting protein synthesis than ricin A chain. Peptides like ricin and abrin are composed of two chains, a A chain which is the toxic unit and a B chain in which acts by binding to cells. Unlike ricin and abrin, gelonin is composed of a single chain, and, because it lacks a B chain for binding to cells, it is itself relatively non-toxic to intact cells (Stirpe, et al. J. Biol. Chem. 255:6947-6953 (1980)). Mammalian cells apparently lack the ability to bind and/or to internalize the native gelonin molecule. Conjugates of gelonin with a tumor-targeting monoclonal antibody, such as the monoclonal antibody ZME directed to an antigen present on certain tumor cells such as melanoma cells, provide both a specific method for binding the gelonin to the cell and a route for internalization of the gelonin antibody complex. One of the advantages of using the toxin gelonin over using toxins such as ricin A chain is this reduced toxicity to normal tissues as compared to the ricin A chain. Gelonin-coupled with a monoclonal antibody directed to an anti-tumor associated antigen is an active and selective immunotoxic agent for tumor therapy.
Several investigators have utilized gelonin as a cytotoxic agent chemically attached to monoclonal antibodies or to peptide hormone cellular targeting ligands. However, chemical modification of gelonin and cellular targeting moieties can reduce targeting efficiently and cytotoxic potential of gelonin itself. Furthermore, natural sources of gelonin are subject to variability in harvesting and plant growth which can affect gelonin cytotoxic activity. The ability to produce a synthetic gelonin toxin, chemically or utilizing recombinant technology, provides a plentiful, reproducible source of the toxin. Therefore, it is highly desirable to prepare a synthetic gene for gelonin and methods for preparation of the synthetic gene using recombinant technology.